1. Field of the Invention
The present invention relates to a detection system and more particularly to a hybrid tracking loop detector system for demodulation of phase shift keyed messages into their coded digital format.
2. Description of the Prior Art
Prior methods for demodulating phase shift keyed messages when frequency and phase are not known at the receiver have included differential detection. In differential detection the phase of a binary bit having a duration of seven periods of the carrier frequency, for example, is compared with the preceding bit in a phase detector. In this system the binary zero is defined, for example, as the phase relationship of 0.degree. .+-. 90.degree. between adjacent bits and a binary one as 180.degree. .+-. 90.degree.. This detection method works well if the delay line length is precisely equal to the received bit length. However, if the received frequency is shifted by an unknown amount the bit length will not match the delay line length. This may be caused by, for example, a large doppler effect as would occur with acoustic telemetry signals transmitted through the water from a high speed vehicle. At doppler shifts of greater than .+-. 1% the resultant phase error seriously reduces the signal-to-noise ratio at the output of the detector, thereby degrading the probability of correct detection.
The present invention overcomes these disadvantages by providing a hybrid tracking loop detection system that operates at very low signal-to-noise ratios and eliminates cycle slipping when there are phase shifts in the received signals.